Liquid fertilizers, in general, have some advantages over solid fertilizers in that they can be handled more easily and more readily applied in controlled amounts to soils. Liquid fertilizers are especially advantageous in areas which are irrigated because the fertilizer can readily be applied to the soil through the irrigation system. For economics in handling, storing, and transporting liquid fertilizers it is desirable for the fertilizer to have a high concentration of plant food components. However, highly concentrated liquid fertilizers have tended to salt-out at low temperatures, which causes problems in storage and in distributing the fertilizers. The problems of salt-out are accentuated by the difficulty of redissolving the solids which have formed.
Attempts to alleviate the problems caused by salt-out have been made in the prior art. For example, in U.S. Pat. No. 4,315,763 Stoller describes a high analysis fertilizer reported therein not to salt-out at 0.degree. C. In U.S. Pat. No. 4,388,101 Lowder describes a liquid fertilizer made from sulfuric acid, urea, and ammonia which is stated to be crystal-free when stored for one month at 32.degree. C. However, liquid fertilizers having salt-out temperatures significantly lower than 32.degree. F. are desirable.
In many areas of the country which are irrigated the soils are alkaline, and the preferred fertilizers for these soils have a low pH in order to increase the acidity of the soil. Sulfuric acid is a conventional component used to achieve a desirable pH for the fertilizer. However, sulfuric acid is corrosive to materials such as mild steel used in storing and dispensing fertilizers and efforts have been made to inhibit the corrosive effect of fertilizers based on sulfuric acid. However, at least some of these efforts have required the use of additives which function as corrosion inhibitors. For example, in U.S. Pat. No. 4,402,852, Young discloses the use of a dialkylthiourea as a corrosion inhibitor for sulfuric acid-urea compositions in the presence of carbon steel, and in U.S. Pat. No. 4,404,116, Young discloses the use of a cupric ion-containing compound for reducing the corrosive effect of urea-sulfuric acid mixtures on stainless steel.
A further problem in the manufacture of fertilizers using sulfuric acid arises from the exothermic nature of its reaction with other components used to make the fertilizer. The exothermic reaction can result in unacceptably high temperatures during the process of making the fertilizer.
This problem has been attacked in several different ways in the prior art. For example, in U.S. Pat. No. 4,310,343, Verdegaal et al discloses the use of a heat sink to dissipate heat which builds up when making a liquid fertilizer having a high nitrogen and sulfur content, and in U.S. Pat. No. 4,445,915, Young discloses a method of removing heat generated by the reaction of urea and sulfuric acid by cooling the liquid phase by direct heat exchange with air.
The prior art discloses teachings which have been used to overcome problems arising (1) from the use of concentrated liquid fertilizers and (2) from the use of sulfuric acid in making the fertilizers; however, highly concentrated acidic fertilizers which have a very low salt-out temperature and which are substantially non-corrosive are desirable.